The present invention relates to an improved sail craft. In particular, the invention relates to a wind powered sailing craft with improved speed performance as compared with the prior art.
Sail powered craft have been well known for many years and have been used for many purposes including commercial and military applications. In more recent times, with the advent of active propulsion systems, wind powered sail craft have generally been restricted to leisure activities.
Popular forms of modem day sail craft include yachts, catamarans and sail boards. Whilst the applications for this type of craft have become more restricted in recent times, there is still a great deal of,interest for leisure applications. The leisure market is substantial and the competition for new and improved designs is significant.
In particular, there is substantial competition to produce a sail craft with superior speed performance as compared with prior art designs. In this regard, the competition to produce sail craft of ever improved speed performance is similar to the competition to produce solar powered or man powered vehicles of greater performance than their predecessors. A notable example of this type of craft that has been designed to produce the best known speed performance is the Australian designed xe2x80x9cYellow Pages Endeavourxe2x80x9d which is a wind powered sail craft that has recorded an average top sailing speed of 46.52 knots in a 19 knot true wind speed with minimal wave height.
However, the xe2x80x9cYellow Pages Endeavourxe2x80x9d is restricted in that it has a reduced handling capability as compared with generally available craft. The most significant of these is that the craft can only sail on one tack.
The present invention is intended to provide a wind powered sail craft with superior speed performance as compared with the prior art. In addition, it is also intended to provide a wind powered sail craft that provides an improved speed performance without sacrificing handling capabilities as generally occurs in the prior art.
Some of the basic nomenclature used throughout the specification is introduced with reference to FIG. 1 that sets out the fundamental principles of craft velocity in relation to true wind velocity. In particular, FIG. 1 diagrammatically represents the theoretical maximum craft velocity that can be achieved with any type of craft. An analysis of FIG. 1 produces a number of relationships that are plotted in FIG. 2.
FIG. 1 is a vector diagram detailing a locus of all possible velocities of a sail craft, designated V, for a given true wind velocity, designated VT, and apparent wind angle, designated xcex2. The velocity of the craft can be projected into a downwind and an upwind component with the maximum downwind and upwind velocities achievable designated VD and VU respectively.
The apparent wind velocity is designated VA. For a given value of true wind VT and apparent wind angle xcex2, the range of all possible craft velocities comprises an arc of a circle with the true wind velocity being a chord. The arc representing all possible craft velocities is designated VPOSS. The maximum possible craft velocity occurs when the velocity V intersects the centre of the circle VPOSS and extends over the diameter of the circle. At this position, the maximum velocity achievable is designated Vmax. As the circle VPOSS designates the range of all possible craft velocities it can be readily seen that the maximum upwind component of velocity VU and downwind component of velocity VD, are projections from the circle of VPOSS parallel to the true wind velocity VT.
From the vector diagram of FIG. 1, it can be readily derived that the maximum speed, Vmax, is given by:       V    max    =            V      T              sin      ⁢              xe2x80x83            ⁢      β      
The maximum velocity made good to windward, that is upwind component, VU, is given by       V    U    =                    V        max            -              V        T              2  
The maximum velocity made good downwind, VD, is given by       V    D    =                    V        max            +              V        T              2  
The boat speed associated with VU is given by   V  =                    sin        ⁢                  (                                    π              4                        -                          β              2                                )                            sin        ⁢                  xe2x80x83                ⁢        β              ⁢          V      T      
The corresponding apparent wind is given by       V    A    =                    sin        ⁢                  (                                    π              4                        +                          β              2                                )                            sin        ⁢                  xe2x80x83                ⁢        β              ⁢          V      T      
The corresponding ratio of boat speed to apparent wind speed is given by             V      A        V    =                    1        +                  sin          ⁢                      xe2x80x83                    ⁢          β                            1        -                  sin          ⁢                      xe2x80x83                    ⁢          β                    
These relationships are plotted for varying apparent wind angle xcex2 and appear in FIG. 2. The vertical axis of the plot in FIG. 2 represents units of true wind speed with one unit representing the true wind speed. The horizontal axis represents varying apparent wind angle from 0 degrees to 90 degrees.
As can be seen from the plots in FIG. 2, the plot representing the maximum velocity of the craft, Vmax, has a value approaching the limit of the true wind speed as the apparent wind angle approaches 90 degrees and that the maximum velocity increases with a decreasing apparent wind angle. The xe2x80x9cYellow Pages Endeavourxe2x80x9d achieved a top speed of approximately 2.5 times the true wind speed on the day of the test, and as can be seen from the plot, this represents an apparent wind angle of approximately 25 degrees.
The true wind angles, designated xcex3, for maximum velocity and maximum up wind and down wind components are as follows:
Vmax is achieved at   γ  =            π      2        +    β  
VU is achieved at   γ  =            π      4        +          β      2      
VD is achieved at   γ  =                    3        ⁢                  xe2x80x83                ⁢        π            4        +          β      2      
However, it is very difficult to obtain low values of apparent wind angle with a sail craft whilst at the same time being able to sail and control the craft.
The analysis presented above and the diagrammatic representations of FIGS. 1 and 2 are applicable to all types of sailing craft. They effectively represent the theoretical principles that apply irrespective of the structure of the craft.
Whilst the above analysis is generally applicable to any type of craft, the following discussion will focus upon the general principles relating to the structure of crafts and leads to a detailed discussion of the specific structure of the craft of the present invention.
In the design of high performance sailcraft it is necessary to consider three principle classes of force, namely aerodynamic, hydrodynamic and gravitational. Hydrostatic forces may be considered to be negligible once the craft has sufficient speed. The resultant of the gravitational forces is a single force acting through the centre of mass. The aerodynamic forces can be reduced to a single resultant force and possibly a residual torque with an axis parallel to the line of action of the resultant force. A similar reduction also applies to the hydrodynamic forces. Ideally the residual torques will be negligible, leaving just the resultant aerodynamic, hydrodynamic and gravitational forces to consider. If three non parallel forces act on a rigid body, then for equilibrium the forces must sum to zero, must be coplanar and must be concurrent.
Additionally, it has been recognised for some time that the analysis of the operation of sail craft can be considered from the perspective of considering the water and the air as two interfacing fluids of substantially different density. As such, sailing craft reside at the interface of the two fluids and impinge into the fluids; the hydrofoil extending into the water and the aerofoil extending into the air. Exploiting this interface is effectively the basis of the operation of sailing craft.
In most conventional sailing craft designs, the hydrofoil and the aerofoil are in generally vertical alignment. In the case of a yacht, the keel forms the hydrofoil and the sail forms the aerofoil. In this instance, the analysis of the various forces acting upon the vessel to produce the motion of the vessel is relatively straightforward as most of the forces acting upon the hydrofoil and aerofoil lie substantially parallel to the horizontal plane of the interface between the two fluids. As will be appreciated by those with a basic understanding of vector addition, the task of analysing resultant forces is greatly simplified if the forces can be represented within a single plane. It is conventional to consider healing moments independently. Pitching moments are often not considered formally. Support of the craft""s weight is also considered independently for both low and high performance craft, which are supported by hydrostatic or dynamic forces, respectively.
Alternative sailing craft designs have been proposed that do not maintain the standard generally vertical alignment between the hydrofoil and the aerofoil. However, it appears to the applicant, with respect to prior art designs that do not have a generally aligned hydrofoil and aerofoil, that there have been limitations in the analysis of forces and the interactions of forces upon the sailing craft. This failure to fully analyse the interacting forces has led to a failure to correctly understand the operation of those forces and hence a failure to optimise the performance of the craft.
In particular, the applicant has recognised that for a correct analysis of the forces acting upon a sailing craft, it is important to consider the forces projected onto the interface (ie. the horizontal plane) as well as the actual forces acting on the craft. For conventional designs that have their actual forces substantially parallel to the horizontal plane the conventional analysis has been correct for the structure of the craft. However, when deviating from conventional structures, the failure to recognise this important aspect leads to non-optimal structural designs.
In the present invention, the applicant has applied the recognition of the need to consider the projection of forces onto the horizontal plane to the analysis of the structure of sailing craft, and has developed an improved sail craft as compared with the prior art.
As part of this recognition, the applicant realised that to effect an improved structural design, various components of the craft would require various degrees of freedom. Accordingly, and unlike the xe2x80x9cYellow Pages Endeavourxe2x80x9d, the improved sail craft of the present invention can sail on both tacks. As a result of this analysis, the applicant has developed a sailing craft with theoretically improved performance as compared with the prior art without sacrificing the ability to sail on both tacks.
The invention provides a wind powered craft including a single hydrofoil assembly, an aerofoil assembly and a hull, with a rigid beam interconnecting the hydrofoil assembly, the aerofoil assembly and the hull, wherein the hull is separate and displaced from the hydrofoil assembly and is, in use, supported above the water by the rigid beam.
It is preferred to connect the hydrofoil assembly, the aerofoil assembly and the hull such that the hydrofoil assembly and the aerofoil assembly are disposed at opposite ends of the rigid beam and the hull is connected to the beam at a position therebetween. It is further preferable that, in use, the aerofoil assembly resides downwind from the hydrofoil assembly.
Preferably the hull is connected to the rigid beam such that when supported above the water, the hull is able to freely rotate about a generally vertical axis. Without any direct control of the yaw motion of the hull, the hull will, when supported above the water, adopt an orientation dependent upon the airflow past the hull. However, the craft may include a rudder or rudders connected to the hull to stabilise yaw motion of the hull. The hull may also include a boom to which a rudder or rudders are connected. It is also preferred that the hydrofoil assembly include a hydrofoil member that, in use, is capable of rotation about an axis generally aligned with the flow of water past the hydrofoil member and the aerofoil assembly include an aerofoil member that, in use, is capable of rotation about an axis generally aligned with the flow of air past the aerofoil member.
Additionally, it is preferred that the hydrofoil member be capable, in use, of rotation about an axis generally transverse to the flow of water past the hydrofoil member, the axis also being generally aligned with the lateral axis of the hydrofoil member. It is also preferable that the aerofoil member be capable, in use, of rotation about an axis generally transverse to the flow of air past the aerofoil member, the axis also being generally aligned with the lateral axis of the aerofoil member.
As well as free rotation of the hull about a generally vertical axis, it is preferred that the hydrofoil assembly and the aerofoil assembly be connected to the rigid beam such that, in use, they may each rotate freely about a generally vertical axis such that the lateral axes of the hydrofoil and aerofoil members are maintained generally transverse to the flow of water or air passing the foils.
In a preferred embodiment, the hydrofoil assembly includes a hydrofoil boom and stabilising foils attached thereto, the hydrofoil boom being fixedly attached to the assembly and extending downstream of the hydrofoil member and assisting to maintain the hydrofoil member lateral axis generally transverse to the flow of water passing the hydrofoil member and acting to stabilise yaw movements of the hydrofoil assembly.
In addition, in a preferred embodiment, the aerofoil assembly includes an aerofoil boom and stabilising foils attached thereto, the aerofoil boom being fixedly attached to the assembly and extending downwind of the aerofoil member and assisting to maintain the aerofoil member lateral axis generally transverse to the flow of air passing the aerofoil member and acting to stabilise yaw movements of the aerofoil assembly.
To reduce hydrodynamic drag, it is preferred that the hydrofoil member be separate and displaced from the connection between the rigid beam and the hydrofoil assembly. However, in addition to avoiding immersion of the connection it is preferable that the axes representing rotation of the hydrofoil assembly about a generally vertical axis, and rotation of the hydrofoil member about an axis generally aligned to the flow of water past the hydrofoil member intersect.
In one embodiment, the hull includes a rudder disposed rearwardly and upwardly from the hull, and in another embodiment, the hull includes a rudder disposed rearwardly and downwardly from the hull. In yet a further embodiment, the hull includes a rudder disposed rearwardly and upwardly from the hull and a rudder disposed rearwardly and downwardly from the hull. In this particular embodiment, the rudder disposed rearwardly and upwardly and the rudder disposed rearwardly and downardly from the hull are capable, in use, of being independently controlled.
In a particularly preferred embodiment, the hull includes float members attached thereto to provide stability to the hull whilst resting upon the surface of the water.
The stabilising foils attached to the foil booms and the hull may include generally horizontally aligned foils to contribute to the control of the pitch of the rigid beam. To a lesser extent, these stabilising foils may also assist roll stabilisation of the rigid beam. Pitch of the rigid beam will also be stabilised by the position of the centre of gravity being below the straight line joining the hydrodynamic centre of pressure and the aerodynamic centre of pressure. Accordingly, it is preferable that the centre of gravity of the craft reside below a straight line projected between the hydrodynamic and aerodynamic centres of pressure.
To gain improved performance, it is preferable to construct the craft such that the angle between the horizontal plane and the straight line joining the hydrodynamic centre of pressure and the aerodynamic centre of pressure, when in use, is as small as possible. Of course, this will impact upon other constraints in relation to the physical dimensions of remaining aspects of the craft in particular the span of the aerofoil and the width of the rigid beam. With respect to the foil assemblies, it is preferable to construct the foils such that at least one of the foils has a wide range of coefficient of lift.
To reduce drag, it is preferred that all elements of the craft be streamlined in accordance with aero and hydrodynamic principles. In particular, it is preferable that the rigid beam has a streamlined cross section to reduce drag forces imparted to the craft.
In a particularly preferred embodiment, the rigid beam comprises two distinct joined sections with an obtuse angle extending between the sections with the hull attached to the beam in the vicinity of the join, the section of the rigid beam connecting the hull to the aerofoil assembly including an aerodynamically shaped cowling or cover that extends for a substantial length along the longitudinal axis of that section of the beam with the cowling capable of rotation about the longitudinal axis of the beam such that it may adopt a position corresponding to the least aerodynamic drag. The orientation of the cowling will therefore depend upon the prevailing wind conditions during use and upon the tack. The section of the rigid beam connecting the hull to the hydrofoil assembly may also have a similar shaped cowling extending for a substantial length of that section. Alternatively this cowling could be symmetrical and fixed.
In another embodiment of the invention, the aerofoil member comprises a flexible and resilient member that is capable, in use, of twisting about an axis generally transverse to the flow of air past the aerofoil member, the axis also being generally aligned with the lateral axis of the aerofoil member.
In a particularly preferred embodiment, the aerofoil member is constructed from two substantially similar members that are capable of independent rotation about their lateral axes. Independently controlled rotation of the aerofoil members about their lateral axes enables rotation of the aerofoil members about an axis generally aligned with the flow of air past the aerofoil members to be effected. In this embodiment it is also preferable to include a hydrofoil member that is constructed from two substantially similar members that are capable of independent rotation about their lateral axes. Independently controlled rotation of the hydrofoil members about lateral axes enables rotation of the hydrofoil members about an axis generally aligned with the flow of water past the hydrofoil members to be effected.